Hi Ken,
Looking at the low temperature side: the limiting factor on the gas for
low-temperature VT is the difference between the gas dew point and the
chiller compressor temperature. Bruker quotes a dew point of no more than
-51 °C for operating a BCU-I, and a dew point of no more than -80 °C for
operating a BCU-II. If you're running the BCU in Flush/Off for experiments
at RT and above, the dew point requirement is substantially relaxed. We have
found that air with a dew point below -80 °C is more than satisfactory for
running -80 °C chillers for at least two weeks at a time; we cycle off our
chillers during our biweekly nitrogen fills. This is for MAS experiments
using both BCU-II units and AirJet chillers. Below -80 °C VT temperatures
you're going to need nitrogen gas.
We've trialed an in-line desiccant compressed air dryer (product number 1156K28
from McMaster-Carr) for when we were having issues with the building dryer. It's
been generally satisfactory, though it wouldn't be enough to get down to -80 °C
by itself. Before going out and buying an in-line dryer, it would definitely be
worth confirming the dryness of your existing house air, if you haven't already;
your university steamfitter shop probably has a portable dew point meter. I
would also caution that particulate filters must be installed between desiccant
dryers and any NMR-relevant equipment.
In short: because of the dryness of our compressed air, we use dry compressed
air for MAS experiments down to sample temperatures of about -50 °C, which
requires VT setpoints of about -60 °C in most cases.
As to high temperatures, when I was doing high-temp MAS experiments I would
change to nitrogen boil-off gas for both VT and spin gas around 80 °C.
Even with dry air I'd be very reluctant to go higher. I've been lucky enough
to always have house nitrogen, so unfortunately I can't offer setup suggestions.
I will note that Bruker two-bay consoles that have an SPB in their BSMS
typically allow for two gas inputs; one gas will be used for flush, VT, and
spin SB, while the other used for shim cooling, lift, and emergency lift.
Separating the two does require some reconfiguration at the back of the
console (which cannot be done while experiments are running), but it's not
overly challenging.
Best of luck,
Alex
Alex Paterson
The National Magnetic Resonance Facility at Madison (NMRFAM)
University of Wisconsin-Madison
alpaterson_at_wisc.edu<mailto:alpaterson_at_wisc.edu>
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Received on Tue Jun 03 2025 - 14:28:32 MST